This invention relates to an improved method of gasifying carbonaceous material such as coal, coke, pitch, and the like (hereunder collectively referred to as "carbonaceous material") by blowing the carbonaceous material together with a gasifying agent such as oxygen onto a molten iron bath at high temperatures.
It is known in the art that a carbonaceous material is injected into a molten iron bath together with a gasifying agent to carry out gasification of the carbonaceous material. This process is called "molten iron coal gasification process". This process is classified into two types: one is a top-blowing process in which carbonaceous material is blown simultaneously with a gasifying agent onto a molten iron bath from the above through one or more top-blowing lances (See U.S. Pat. Nos. 4,388,084 and 4,389,246); the other one is a bottom-blowing process in which the carbonaceous material is blown simultaneously with a gasifying agent onto the molten iron bath from a tuyere provided under the surface of the molten metal bath (See U.S. Pat. Nos. 3,533,739 and 3,526,478). It has been thought that the top-blowing process is more advantageous than the bottom blowing process in its gasification efficiency, properties of the produced gas and operational stability.
Namely, so long as the top-blowing process is concerned, there is no leakage of molten iron from the tuyeres and the blowing can be stopped immediately without having to worry about the clogging of the tuyeres, even when a trouble occurs in the blowing system. In contrast, in the case of the bottom-blowing process the tuyeres would easily be clogged if the blowing was stopped when an accident occurred in the blowing system.
The top-blowing process is also superior to the bottom-blowing process in its gasification efficiency, i.e. the amount of carbonaceous material gasified per unit treating time, since the bottom blowing process has an inherent upper limit in the blowing rate of a carrier gas for carbonaceous material. The upper limit is determined on the depth of a molten iron bath employed. If the blowing rate increases above the upper limit, unreacted coal is blown off through the molten iron bath, markedly decreasing the efficiency of gasification. On the other hand, a lower limit also exists to prevent the clogging of tuyeres. Thus, the blowing rate of a carrier gas of the bottom blowing process is restricted to within a relatively small range.
In contrast, according to the top-blowing process, the process is free from the clogging of the tuyeres or the passing through of the carbonaceous material. The top-blowing process is not limited, in practice, in respect to the blowing rate of carbonaceous material, either. Thus, according to the top-blowing process, the volume of the gas produced per unit treating time is very large and it is easy to control the volume, i.e. productivity.
However, the top-blowing process has a lot of heat balance problems common to all other coal gasification processes with a molten iron bath, although they have many advantages such as in the above.
Namely, according to the top-blowing process the carbonaceous material is decomposed at fire points the temperatures of which are much higher than that required to decompose it inother processes. Thus, the resulting gas of this top-blowing process is rich in CO and H.sub.2, and the proportion of CO.sub.2 is rather small. This means that such a gas composition as in the above is satisfactory to be utilized as a fuel gas and as a chemical raw material. But, this also means that the carbon added is converted into CO gas, not to CO.sub.2 gas. The conversion into CO.sub.2 gas generates heat enough to promote gasification. In the case of the gasification of coal containing a large amount of ash, moisture and volatile matters, the thermal balance of the top blowing gasification process shifts itself to an endothermic one, making continuation of the process quite difficult. In order to cope with these problems, there has been proposed the following two methods:
One method is to combine a highly exothermic, high grade coal with the above low grade coal to provide a mixture containing less ash, moisture and volatiles. The thus combined mixture of coal is then subjected to gasification. However it is quite expensive to keep a constant mixing ratio, and to keep the coal composition constant throughout the process. Even the mere employment of pulverization and mixing adds to the manufacturing cost significantly.
It is generally said that a coal gasification plant should be built in an area where coal is mined so as to reduce the gasification costs. However, it is low grade coal which is highly demanded today for being treated through gasification processes in order to increase the utility value of the products. Since this type of material is less expensive, a commercial gasification plant is feasible. However, in practice, stable operation cannot be achieved on a commercial basis, because it is quite rare that high grade coal and low grade coal are found at the same mining site. For the above reasons, it is impractical to balance the thermal conditions by means of combining a less exothermic, low grade coal with a highly exothermic, high grade coal.
The other method is the one called the "soft blowing" method, in which a secondary combustion is carried out by means of increasing the height of the lance, i.e. the distance between the nozzle end of the lance and the surface of the molten iron bath.
Namely, the carbonaceous material is injected through the lance to reach the molten iron bath surface and then goes into the melt. Since according to this secondary combustion method, the height of the lance is increased, the distance between the lance tip and the molten iron bath surface is also increased, and the time the carbonaceous material takes to go from the lance to the molten metal surface is also increased. This means that before the carbonaceous material reaches the surface of the molten metal bath, it reacts with a gasifying agent such as oxygen and the amount of sulfur which is carried in the combustion gas is markedly increased in comparison with the amount of sulfur which is caught by the slag placed on the molten metal bath. This results in an increase in the sulfur content of the product gas. A desulfurization apparatus has to be installed to treat the product gas to decrease the sulfur content to a feasible level. This also adds to the manufacturing costs of the product gas.